When it's tired out, and in the net bonk it between the eyes ONCE good. Stick a small rope/stringer (which is attached to a cleat or whatever) through its's mouth and out the
gill(s) cut gills with scissors or knife and/or cut throat area to bleed. Put boat in gear and drag it for a minute while re-rigging or setting your crap back to the depth you dangled.

You can drag it in the net or if your rope/stringer is attached to a cleat drag it there.
When satisfied that it is bled well, lift by rope/stringer and slice the belly either anal vent forward or commercial style (Just slit the belly open and yank out what you can easily grab ). I like anal vent forward method then I can easily scrape the blood out along the spine. Voila! Straight to ice/cooler.
No mess, except a little on the exterior water line, no biggy.
Keeps more scales on it (prettier for photos and when showing the wife why I 'waste' all my time and $$ fishing!) it also doesn't flop on floor spewing blood everywhere.

I used to do it all in the motor-well but that got messy and I didn't like the idea of oil and gas residue on my fish.

A million ways to skin a cat, or a salmon....